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class="dropdown-item"><!----> <a href="/pages/28ec23/" class="nav-link">NAND Flash</a></li><li class="dropdown-item"><!----> <a href="/pages/62bf40/" class="nav-link">ARM 处理器</a></li><li class="dropdown-item"><!----> <a href="/pages/1a9374/" class="nav-link">嵌入式基础知识-存储器</a></li><li class="dropdown-item"><!----> <a href="/pages/aac5e3/" class="nav-link">闪存存储和制造技术概述</a></li><li class="dropdown-item"><!----> <a href="/pages/8f6056/" class="nav-link">芯片IO驱动力</a></li><li class="dropdown-item"><!----> <a href="/pages/d146b8/" class="nav-link">主流先进封装技术介绍</a></li><li class="dropdown-item"><!----> <a href="/pages/16f0ba/" class="nav-link">NAND Flash基础</a></li><li class="dropdown-item"><!----> <a href="/pages/90d8d0/" class="nav-link">基于PA算法的FTL引导</a></li><li class="dropdown-item"><!----> <a href="/pages/eb672b/" class="nav-link">SD逻辑擦除和物理擦除</a></li><li class="dropdown-item"><!----> <a href="/pages/747121/" class="nav-link">NAND Flash的SDR、ONFI、DDR接口</a></li><li class="dropdown-item"><!----> <a href="/pages/1eb351/" class="nav-link">【详解】Nand Flash必看知识</a></li><li class="dropdown-item"><!----> <a href="/pages/d2512a/" class="nav-link">【两万字详解】Nand Flash必看知识</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="面经" class="dropdown-title"><!----> <span class="title" style="display:;">面经</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/d69946/" class="nav-link">虎牙C++技术面经</a></li><li class="dropdown-item"><!----> <a href="/pages/29251d/" class="nav-link">金山一面复习</a></li><li class="dropdown-item"><!----> <a href="/pages/c7c01f/" class="nav-link">完美世界秋招 C++ 游戏开发面经(Cpp部分)</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="其它" class="dropdown-title"><!----> <span class="title" style="display:;">其它</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/fa256e/" class="nav-link">博客搭建</a></li><li class="dropdown-item"><!----> <a href="/pages/335531/" class="nav-link">网站收藏箱</a></li></ul></div></div> <!----></nav></div></header> <div class="sidebar-mask"></div> <div class="sidebar-hover-trigger"></div> <aside class="sidebar" style="display:none;"><!----> <nav class="nav-links"><div class="nav-item"><a href="/" class="nav-link">首页</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="CPP语言" class="dropdown-title"><!----> <span class="title" style="display:;">CPP语言</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/c5bdd8/" class="nav-link">Cpp之旅</a></li><li class="dropdown-item"><!----> <a href="/pages/279e62/" class="nav-link">Cpp专栏</a></li><li class="dropdown-item"><!----> <a href="/pages/801755/" class="nav-link">Effective_CPP</a></li><li class="dropdown-item"><!----> <a href="/pages/6b2468/" class="nav-link">muduo网络库</a></li><li class="dropdown-item"><!----> <a href="/pages/5f8c9f/" class="nav-link">Unix环境高级编程</a></li><li class="dropdown-item"><!----> <a href="/pages/3f1d21/" class="nav-link">Cpp提高编程</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="计算机基础" class="dropdown-title"><!----> <span class="title" style="display:;">计算机基础</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/7b1cb2/" class="nav-link">计算机网络</a></li><li class="dropdown-item"><!----> <a href="/pages/6048a8/" class="nav-link">操作系统</a></li><li class="dropdown-item"><!----> <a href="/pages/3b34ba/" class="nav-link">数据结构</a></li><li class="dropdown-item"><!----> <a href="/pages/412fe7/" class="nav-link">Linux</a></li><li class="dropdown-item"><!----> <a href="/pages/2dcfa1/" class="nav-link">算法</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="数据库" class="dropdown-title"><!----> <span class="title" style="display:;">数据库</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/efa3f2/" class="nav-link">基础篇</a></li><li class="dropdown-item"><!----> <a href="/pages/ccc445/" class="nav-link">MySql</a></li><li class="dropdown-item"><!----> <a href="/pages/54616e/" class="nav-link">Redis</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="嵌入式软件开发" class="dropdown-title"><!----> <span class="title" style="display:;">嵌入式软件开发</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/d142c2/" class="nav-link">电子嵌入式通信协议</a></li><li class="dropdown-item"><!----> <a href="/pages/4c6bf3/" class="nav-link">深入浅出SSD</a></li><li class="dropdown-item"><!----> <a href="/pages/d3f36a/" class="nav-link">文件系统</a></li><li class="dropdown-item"><!----> <a href="/pages/e0cca7/" class="nav-link">汇编语言</a></li><li class="dropdown-item"><!----> <a href="/pages/fab2d7/" class="nav-link">STM32</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="开发日常" class="dropdown-title"><!----> <span class="title" style="display:;">开发日常</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/e472d1/" class="nav-link">随笔（持续更新）</a></li><li class="dropdown-item"><!----> <a href="/pages/71f6ae/" class="nav-link">Git知识总结</a></li><li class="dropdown-item"><!----> <a href="/pages/db6fb8/" class="nav-link">Git备忘清单</a></li><li class="dropdown-item"><!----> <a href="/pages/e1081f/" class="nav-link">Git 创建删除远程分支</a></li><li class="dropdown-item"><!----> <a href="/pages/777b8a/" class="nav-link">nvm使用小结</a></li><li class="dropdown-item"><!----> <a href="/pages/ee770e/" class="nav-link">虚拟机固定 IP 地址</a></li><li class="dropdown-item"><!----> <a href="/pages/1ab9a6/" class="nav-link">Shell 脚本学习笔记</a></li><li class="dropdown-item"><!----> <a href="/pages/411aa4/" class="nav-link">VScode 插件 CodeGeeX 使用教程</a></li><li class="dropdown-item"><!----> <a href="/pages/0d525d/" class="nav-link">KylinV10 将项目上传至 Github教程</a></li><li class="dropdown-item"><!----> <a href="/pages/907786/" class="nav-link">KylinV10 安装 MySQL 教程（可防踩雷）</a></li><li class="dropdown-item"><!----> <a href="/pages/a2d21e/" class="nav-link">kylinV10-SP1 安装 QT</a></li><li class="dropdown-item"><!----> <a href="/pages/b561cf/" class="nav-link">高并发内存池</a></li><li class="dropdown-item"><!----> <a href="/pages/6ab6d1/" class="nav-link">USBGUARD 项目编译环境配置</a></li><li class="dropdown-item"><!----> <a href="/pages/883f02/" class="nav-link">Power_Destory 项目</a></li><li class="dropdown-item"><!----> <a href="/pages/479472/" class="nav-link">U 盘清除工具编译教程</a></li><li class="dropdown-item"><!----> <a href="/pages/9c4241/" class="nav-link">个人博客代码推送教程</a></li><li class="dropdown-item"><!----> <a href="/pages/3ad765/" class="nav-link">SVN Trunk Branches的Merge操作</a></li><li class="dropdown-item"><!----> <a href="/pages/0c0ca8/" class="nav-link">如何高效阅读嵌入式项目代码</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端学习" class="dropdown-title"><!----> <span class="title" style="display:;">前端学习</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/99897f/" class="nav-link">HTML与CSS</a></li><li class="dropdown-item"><!----> <a href="/pages/51542d/" class="nav-link">JS学习</a></li><li class="dropdown-item"><!----> <a href="/pages/803f9d/" class="nav-link">Vue3入门</a></li><li class="dropdown-item"><!----> <a href="/pages/ca4cfb/" class="nav-link">Vue3进阶</a></li><li class="dropdown-item"><!----> <a href="/pages/50e8d3/" class="nav-link">黑马Vue3</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="MFC" class="dropdown-title"><!----> <span class="title" style="display:;">MFC</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/a4b108/" class="nav-link">MFC编程随记</a></li><li class="dropdown-item"><!----> <a href="/pages/41acbd/" class="nav-link">MFC实现ini配置文件的读取</a></li><li class="dropdown-item"><!----> <a href="/pages/951a7a/" class="nav-link">MFC实现点击列表头排序</a></li><li class="dropdown-item"><!----> <a href="/pages/a8598f/" class="nav-link">贴图法美化Button按钮</a></li><li class="dropdown-item"><!----> <a href="/pages/054516/" class="nav-link">MFC使用细节</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="闪存" class="dropdown-title"><!----> <span class="title" style="display:;">闪存</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/b925b8/" class="nav-link">如何高效阅读嵌入式项目代码</a></li><li class="dropdown-item"><!----> <a href="/pages/28ec23/" class="nav-link">NAND Flash</a></li><li class="dropdown-item"><!----> <a href="/pages/62bf40/" class="nav-link">ARM 处理器</a></li><li class="dropdown-item"><!----> <a href="/pages/1a9374/" class="nav-link">嵌入式基础知识-存储器</a></li><li class="dropdown-item"><!----> <a href="/pages/aac5e3/" class="nav-link">闪存存储和制造技术概述</a></li><li class="dropdown-item"><!----> <a href="/pages/8f6056/" class="nav-link">芯片IO驱动力</a></li><li class="dropdown-item"><!----> <a href="/pages/d146b8/" class="nav-link">主流先进封装技术介绍</a></li><li class="dropdown-item"><!----> <a href="/pages/16f0ba/" class="nav-link">NAND Flash基础</a></li><li class="dropdown-item"><!----> <a href="/pages/90d8d0/" class="nav-link">基于PA算法的FTL引导</a></li><li class="dropdown-item"><!----> <a href="/pages/eb672b/" class="nav-link">SD逻辑擦除和物理擦除</a></li><li class="dropdown-item"><!----> <a href="/pages/747121/" class="nav-link">NAND Flash的SDR、ONFI、DDR接口</a></li><li class="dropdown-item"><!----> <a href="/pages/1eb351/" class="nav-link">【详解】Nand Flash必看知识</a></li><li class="dropdown-item"><!----> <a href="/pages/d2512a/" class="nav-link">【两万字详解】Nand Flash必看知识</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="面经" class="dropdown-title"><!----> <span class="title" style="display:;">面经</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/d69946/" class="nav-link">虎牙C++技术面经</a></li><li class="dropdown-item"><!----> <a href="/pages/29251d/" class="nav-link">金山一面复习</a></li><li class="dropdown-item"><!----> <a href="/pages/c7c01f/" class="nav-link">完美世界秋招 C++ 游戏开发面经(Cpp部分)</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="其它" class="dropdown-title"><!----> <span class="title" style="display:;">其它</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/pages/fa256e/" class="nav-link">博客搭建</a></li><li class="dropdown-item"><!----> <a href="/pages/335531/" class="nav-link">网站收藏箱</a></li></ul></div></div> <!----></nav>  <ul class="sidebar-links"><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>Cpp之旅</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/pages/c5bdd8/" class="sidebar-link">第1章 基础</a></li><li><a href="/pages/88a7db/" aria-current="page" class="active sidebar-link">第2章 用户自定义类型</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/pages/88a7db/#_2-1-引言" class="sidebar-link">2.1 引言</a></li><li class="sidebar-sub-header level2"><a href="/pages/88a7db/#_2-2-结构" class="sidebar-link">2.2 结构</a></li><li class="sidebar-sub-header level2"><a href="/pages/88a7db/#_2-3-类" class="sidebar-link">2.3 类</a></li><li class="sidebar-sub-header level2"><a href="/pages/88a7db/#_2-4-枚举" class="sidebar-link">2.4 枚举</a></li><li class="sidebar-sub-header level2"><a href="/pages/88a7db/#_2-5-联合" class="sidebar-link">2.5 联合</a></li><li class="sidebar-sub-header level2"><a href="/pages/88a7db/#_2-6-建议" class="sidebar-link">2.6 建议</a></li></ul></li><li><a href="/pages/61ffe6/" class="sidebar-link">第3章 模块化</a></li><li><a href="/pages/2dfe3d/" class="sidebar-link">第4章 错误处理</a></li><li><a href="/pages/1cb324/" class="sidebar-link">第5章 类</a></li><li><a href="/pages/fae683/" class="sidebar-link">第6章 基本操作</a></li><li><a href="/pages/2cdace/" class="sidebar-link">第7章 模板</a></li><li><a href="/pages/619376/" class="sidebar-link">第8章 概念和泛型编程</a></li><li><a href="/pages/48ac45/" class="sidebar-link">第9章 标准库</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Cpp专栏</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Effetcive_CPP</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>muduo网络库</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Unix环境高级编程</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Cpp提高编程</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><span data-v-06225672>CPP语言</span></li><li data-v-06225672><span data-v-06225672>Cpp之旅</span></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>霜晨月</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2023-11-24</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">第2章 用户自定义类型<!----></h1> <!----> <div class="theme-vdoing-content content__default"><h1 id="第2章-用户自定义类型"><a href="#第2章-用户自定义类型" class="header-anchor">#</a> 第2章 用户自定义类型</h1> <h2 id="_2-1-引言"><a href="#_2-1-引言" class="header-anchor">#</a> 2.1 引言</h2> <ul><li>用基本类型、const修饰符和声明操作符构造出来的类型，称为内置类型。</li> <li>Cpp抽象机制的目的：令程序员能够设计并实现他们自己的数据类型。</li> <li>利用Cpp的抽象机制从其他类型构造出来的类型称为用户自定义类型，即类和枚举。</li></ul> <h2 id="_2-2-结构"><a href="#_2-2-结构" class="header-anchor">#</a> 2.2 结构</h2> <p>构建新类型的第一步通常是：把所需元素组织成一种数据结构。</p> <p>一个struct的例子如下：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">struct</span> <span class="token class-name">Vector</span><span class="token punctuation">{</span>
    <span class="token keyword">double</span><span class="token operator">*</span> elem<span class="token punctuation">;</span>	<span class="token comment">//指向元素的指针</span>
    <span class="token keyword">int</span> sz<span class="token punctuation">;</span>			<span class="token comment">//元素的数量</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token comment">//可通过下述方式定义：</span>
Vector v<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>然而，它本身并无太大用处，因为v的elem指针并不指向任何实际内容。为了变得有用，需令v指向某些元素。</p> <p>例如：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">void</span> <span class="token function">vector_init</span><span class="token punctuation">(</span>Vector<span class="token operator">&amp;</span> v<span class="token punctuation">,</span><span class="token keyword">int</span> s<span class="token punctuation">)</span><span class="token punctuation">{</span>	<span class="token comment">//初始化Vector类型</span>
    v<span class="token punctuation">.</span>elem <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">double</span><span class="token punctuation">[</span>s<span class="token punctuation">]</span><span class="token punctuation">;</span> 		<span class="token comment">//分配数组空间，包含s个double类型的值</span>
    v<span class="token punctuation">.</span>sz <span class="token operator">=</span> s<span class="token punctuation">;</span>						
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>new操作符从名为自由存储（也叫动态内存或者堆）的区域中分配内存。分配在自由存储中的对象作用域与创建时所处的作用域无关，它会一直存活，直到调用delete操作符销毁它为止。</p> <p>Vector的一个简单应用如下：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">double</span> <span class="token function">read_and_sum</span><span class="token punctuation">(</span><span class="token keyword">int</span> s<span class="token punctuation">)</span> <span class="token punctuation">{</span>	<span class="token comment">//从cin读入s个整数，然后返回它们的和；假定s为正</span>
    Vector v<span class="token punctuation">;</span>
    <span class="token function">vector_init</span><span class="token punctuation">(</span>v<span class="token punctuation">,</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">!=</span> s<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span>
    	cin <span class="token operator">&gt;&gt;</span> v<span class="token punctuation">.</span>elem<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>		<span class="token comment">//读入元素</span>
    <span class="token keyword">double</span> sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">!=</span> s<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span>
        sum <span class="token operator">+=</span> v<span class="token punctuation">.</span>elem<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>		<span class="token comment">//计算元素的和</span>
    <span class="token keyword">return</span> sum<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>标准库类型使用小写名称，所以自定义的类型的名称通常使用首字母大写（例如：Vector和String），以示区别。</p> <p><strong>不要试图重新发明vector和string这样的标准库组件；直接使用现成的更明智。</strong></p> <p><strong>访问struct成员有两种方式，通过名字或者引用来访问时用 . （点），通过指针访问时用 -&gt; 。</strong></p> <p>例如：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">void</span> <span class="token function">f</span><span class="token punctuation">(</span>Vector v<span class="token punctuation">,</span> Vector<span class="token operator">&amp;</span> rv<span class="token punctuation">,</span> Vector<span class="token operator">*</span> pv<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">int</span> i1 <span class="token operator">=</span> v<span class="token punctuation">.</span>sz<span class="token punctuation">;</span>		<span class="token comment">//通过名字访问</span>
    <span class="token keyword">int</span> i2 <span class="token operator">=</span> rv<span class="token punctuation">.</span>sz<span class="token punctuation">;</span>		<span class="token comment">//通过引用访问</span>
    <span class="token keyword">int</span> i3 <span class="token operator">=</span> pv<span class="token operator">-&gt;</span>sz<span class="token punctuation">;</span>	<span class="token comment">//通过指针访问</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="_2-3-类"><a href="#_2-3-类" class="header-anchor">#</a> 2.3 类</h2> <p>把类型的接口（所有代码都可使用的部分）与其实现（可访问外部不可访问的数据）分离开来。在Cpp中，实现上述目的的语言机制被称为<strong>类</strong>。</p> <p>类的public成员定义了该类的接口，private成员则只能通过接口访问。public和private成员声明允许以任意顺序出现在类声明中，但是按惯例通常将public声明放在private前面，除非需要特别强调private成员的实现。</p> <p>例如：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">class</span> <span class="token class-name">Vector</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
    <span class="token function">Vector</span><span class="token punctuation">(</span><span class="token keyword">int</span> s<span class="token punctuation">)</span> <span class="token operator">:</span> elem<span class="token punctuation">{</span><span class="token keyword">new</span> <span class="token keyword">double</span><span class="token punctuation">[</span>s<span class="token punctuation">]</span><span class="token punctuation">}</span><span class="token punctuation">,</span> sz<span class="token punctuation">{</span>s<span class="token punctuation">}</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>	<span class="token comment">//构造一个Vector</span>
    <span class="token keyword">double</span><span class="token operator">&amp;</span> <span class="token keyword">operator</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> elem<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span><span class="token punctuation">}</span>	<span class="token comment">//通过下标访问元素</span>
    <span class="token keyword">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">return</span> sz<span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token keyword">private</span><span class="token operator">:</span>
    <span class="token keyword">double</span><span class="token operator">*</span> elem<span class="token punctuation">;</span>	<span class="token comment">//指向元素的指针</span>
    <span class="token keyword">int</span> sz<span class="token punctuation">;</span>			<span class="token comment">//元素的数量</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token comment">//定义一个Vector类型的变量</span>
Vector <span class="token function">v</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>	<span class="token comment">//拥有6个元素的Vector</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><ul><li><p>总的来说，Vector对象是一个“句柄”，它包含指向元素的指针（elem）及元素的数量（sz）。</p></li> <li><p>在这里，我们只能通过Vector的接口访问其表示形式（成员elem和sz）。Vector接口由其public成员构成，包括Vector()、operator<a href=""></a>和size()。</p></li></ul> <p>所以之前的read_and_sum()示例可简化为：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">double</span> <span class="token function">read_and_sum</span><span class="token punctuation">(</span><span class="token keyword">int</span> s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    Vector <span class="token function">v</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span>				<span class="token comment">//创建一个包含s个元素的动态数组</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">!=</span> s<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span>
        cin <span class="token operator">&gt;&gt;</span> v<span class="token punctuation">.</span>elem<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>		<span class="token comment">//读入元素</span>
    <span class="token keyword">double</span> sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">!=</span> s<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span>
        sum <span class="token operator">+=</span> v<span class="token punctuation">.</span>elem<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>		<span class="token comment">//计算元素的和</span>
    <span class="token keyword">return</span> sum<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><ul><li><p>与所属类同名的成员函数被称为构造函数，即它是用来构造类的对象的。与普通函数不同，构造函数在初始化类的对象时一定会被调用。因此定义一个构造函数可以消除类变量未初始化造成的问题。</p></li> <li><p><code>Vector(int s) : elem{new double[s]}, sz{s} { }</code></p></li> <li><p>该构造函数使用成员初始值列表来初始化Vector的成员：</p></li> <li><p>这条语句的含义是：首先从自由存储分配能容纳s个double类型的元素的空间，用指向这个空间的指针初始化elem，然后将sz初始化为s。</p></li></ul> <p><strong>==我们常用的两个关键字struct和class没有本质区别，唯一的不同之处在于，struct的成员默认是public的。例如，我们也可以为struct定义构造函数和其他成员函数，这一点与class完全一致。==</strong></p> <h2 id="_2-4-枚举"><a href="#_2-4-枚举" class="header-anchor">#</a> 2.4 枚举</h2> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">enum</span> <span class="token keyword">class</span> <span class="token class-name">Color</span><span class="token punctuation">(</span> red<span class="token punctuation">,</span> blue<span class="token punctuation">,</span> green<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">enum</span> <span class="token keyword">class</span> <span class="token class-name">Traffic_light</span> <span class="token punctuation">{</span> green<span class="token punctuation">,</span> yellow<span class="token punctuation">,</span> red <span class="token punctuation">}</span><span class="token punctuation">;</span>
Color col <span class="token operator">=</span> Color<span class="token double-colon punctuation">::</span>red<span class="token punctuation">;</span>
Traffic_light light <span class="token operator">=</span> Traffic_light<span class="token double-colon punctuation">::</span>red<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><ul><li>注意：枚举值（例如：red）的作用域在它们的 enum class 内，因此它们可以在不同的 enum class 中重复使用而不会混淆。</li> <li>例如：Color::red 是Color的red值；与Traffic_light::red完全不同。</li> <li>enum后面的class表示这个枚举类型是强类型，并且具备独立作用域。</li></ul> <p>不同的enum class 是不同的类型，这有助于防止对常量的误用。比如：不能混用Traffic_light类与Color类的枚举值</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code>Color x1 <span class="token operator">=</span> red<span class="token punctuation">;</span>					<span class="token comment">//错误：哪个red?</span>
Color y2 <span class="token operator">=</span> Traffic_light<span class="token double-colon punctuation">::</span>red<span class="token punctuation">;</span>	<span class="token comment">//错误：这个red不属于Color类型</span>
Color z3 <span class="token operator">=</span> Color<span class="token double-colon punctuation">::</span>red<span class="token punctuation">;</span>			<span class="token comment">//可行</span>
<span class="token keyword">auto</span> x4 <span class="token operator">=</span> Color<span class="token double-colon punctuation">::</span>red<span class="token punctuation">;</span>			<span class="token comment">//可行：Color::red是Color类型</span>
<span class="token comment">//类似的，也无法隐式地混用Color与整数类型的值</span>
<span class="token keyword">int</span> i <span class="token operator">=</span> Color<span class="token double-colon punctuation">::</span>red<span class="token punctuation">;</span>	<span class="token comment">//错误：Color::red不是int类型</span>
Color c <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>		<span class="token comment">//初始化错误：2不是Color类型</span>
<span class="token comment">//允许显示地指定从int类型进行转换：</span>
Color x <span class="token operator">=</span> Color<span class="token punctuation">{</span><span class="token number">5</span><span class="token punctuation">}</span><span class="token punctuation">;</span>	<span class="token comment">//可行，但烦琐</span>
Color y <span class="token punctuation">{</span><span class="token number">6</span><span class="token punctuation">}</span><span class="token punctuation">;</span>		<span class="token comment">//可行</span>
<span class="token comment">//类似地，可以显示地将enum值转换到其实际存储类型：</span>
<span class="token keyword">int</span> x <span class="token operator">=</span> <span class="token keyword">int</span><span class="token punctuation">(</span>Color<span class="token double-colon punctuation">::</span>red<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>如果不想显式指定枚举地名称，并且希望枚举值的类型直接是int（而不需要显式类型转换），可以去掉enum class中的class字样，得到一个“普通的”enum。普通的enum中枚举值进入与enum自身同级的作用域，并且可以被隐式转换为整数数值。例如：</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">enum</span> <span class="token class-name">Color</span> <span class="token punctuation">{</span>red<span class="token punctuation">,</span> green<span class="token punctuation">,</span> blue<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> col <span class="token operator">=</span> green<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>此处col的值为1。默认情况下，枚举值的整数数值从0开始，逐个加1。</p> <h2 id="_2-5-联合"><a href="#_2-5-联合" class="header-anchor">#</a> 2.5 联合</h2> <p>union是一种特殊的struct，它的所有成员都被分配在同一块内存区域中，因此，union实际占用的空间就是它最大的成员所占的空间。</p> <p>显然，同一时刻，union中只能保存一个成员的值。</p> <ul><li>例如：考虑实现一个符号表的表项，它保存着一个名字和一个值，这个值要么是Node*，要么是int类型，程序可能如下：</li></ul> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">enum</span> <span class="token keyword">class</span> <span class="token class-name">Type</span> <span class="token punctuation">{</span>ptr<span class="token punctuation">,</span>num<span class="token punctuation">}</span><span class="token punctuation">;</span><span class="token comment">//Type 可以是ptr或者num</span>
<span class="token keyword">struct</span> <span class="token class-name">Entry</span> <span class="token punctuation">{</span>
    string name<span class="token punctuation">;</span>
    Type t<span class="token punctuation">;</span>
    Node<span class="token operator">*</span> p<span class="token punctuation">;</span>	<span class="token comment">//如果t == Type::ptr,使用p</span>
    <span class="token keyword">int</span> i<span class="token punctuation">;</span>		<span class="token comment">//如果t == Type::num,使用i</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">f</span><span class="token punctuation">(</span>Entry<span class="token operator">*</span> pe<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>pe<span class="token operator">-&gt;</span>t <span class="token operator">==</span> Type<span class="token double-colon punctuation">::</span>num<span class="token punctuation">)</span>
        cout <span class="token operator">&lt;&lt;</span> pe<span class="token operator">-&gt;</span>i<span class="token punctuation">;</span>
    <span class="token comment">//...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><ul><li>成员p和i从来不会同时被使用，所以空间浪费。使用union可以解决该问题，例如，把两者都定义为union的成员：</li></ul> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">union</span> Value <span class="token punctuation">{</span>
    Node<span class="token operator">*</span> p<span class="token punctuation">;</span>
    <span class="token keyword">int</span> i<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><ul><li><p>对于相同的Value对象而言，现在Value::p和Value::i将被放在相同的内存地址。</p></li> <li><p>因为Cpp语言本身不负责跟踪union实际存储的值的类型，所以需要程序员手动维护如下代码：</p></li></ul> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">enum</span> <span class="token keyword">class</span> <span class="token class-name">Type</span> <span class="token punctuation">{</span>ptr<span class="token punctuation">,</span>num<span class="token punctuation">}</span><span class="token punctuation">;</span><span class="token comment">//Type 可以是ptr或者num</span>
<span class="token keyword">struct</span> <span class="token class-name">Entry</span> <span class="token punctuation">{</span>
    string name<span class="token punctuation">;</span>
    Type t<span class="token punctuation">;</span>
    Value v<span class="token punctuation">;</span>		<span class="token comment">//如果t == Type::ptr,使用v.p;如果t == Type::num,使用v.i</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">f</span><span class="token punctuation">(</span>Entry<span class="token operator">*</span> pe<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>pe<span class="token operator">-&gt;</span>t <span class="token operator">==</span> Type<span class="token double-colon punctuation">::</span>num<span class="token punctuation">)</span>
        cout <span class="token operator">&lt;&lt;</span> pe<span class="token operator">-&gt;</span>i<span class="token punctuation">;</span>
    <span class="token comment">//...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><ul><li><p>时刻维护类型字段（也叫标记，此处为t）与union中实际类型的对应关系并不容易。</p></li> <li><p>也可以使用标准库类型variant，从而消除大多数需要直接使用union的情形。variant保存给定的类型列表集合中的一个值。</p></li></ul> <p>例如，variant&lt;Node* t,int&gt;可以保存Node*或者int类型的值。</p> <div class="language-CPP line-numbers-mode"><pre class="language-cpp"><code><span class="token keyword">enum</span> <span class="token keyword">class</span> <span class="token class-name">Type</span> <span class="token punctuation">{</span>ptr<span class="token punctuation">,</span>num<span class="token punctuation">}</span><span class="token punctuation">;</span><span class="token comment">//Type 可以是ptr或者num</span>
<span class="token keyword">struct</span> <span class="token class-name">Entry</span> <span class="token punctuation">{</span>
    string name<span class="token punctuation">;</span>
    variant<span class="token operator">&lt;</span>Node<span class="token operator">*</span> <span class="token punctuation">,</span><span class="token keyword">int</span><span class="token operator">&gt;</span> v<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">f</span><span class="token punctuation">(</span>Entry<span class="token operator">*</span> pe<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token generic-function"><span class="token function">holds_alternative</span><span class="token generic class-name"><span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&gt;</span></span></span><span class="token punctuation">(</span>pe<span class="token operator">-&gt;</span>v<span class="token punctuation">)</span><span class="token punctuation">)</span>	<span class="token comment">//*pe是否保存了int类型？</span>
        cout <span class="token operator">&lt;&lt;</span> <span class="token generic-function"><span class="token function">get</span><span class="token generic class-name"><span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&gt;</span></span></span><span class="token punctuation">(</span>pe<span class="token operator">-&gt;</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span>		<span class="token comment">//获取这个int</span>
    <span class="token comment">//...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>在很多使用场景中，variant都比union更简单、更安全。</p> <h2 id="_2-6-建议"><a href="#_2-6-建议" class="header-anchor">#</a> 2.6 建议</h2> <ol><li>当内置类型过于底层时，优先使用定义良好的用户自定义类型；</li> <li>将有关联的数据组织为结构（struct或class）；</li> <li>用class表达接口与实现的区别；</li> <li>一个struct就是一个成员默认的public的class；</li> <li>优先使用enum class而不是“普通”enum，以避免很多麻烦；</li> <li>避免使用“裸”union；将其与类型字段一起封装到一个类中；</li> <li>优先使用std::variant，而不是“裸”union;</li></ol></div></div> <!----> <div class="page-edit"><!----> <!----> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2024/6/3 14:54:44</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/pages/c5bdd8/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">第1章 基础</div></a> <a href="/pages/61ffe6/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">第3章 模块化</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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